Economic assessment of a natural gas upgrading process using pressure vacuum swing adsorption for nitrogen removal - Adsorption

The conventional process employed for the removal of N2 in a natural gas upgrading facility is cryogenic distillation (CD) and entails a significant investment outlay. Operation of the CD unit occurs at approximately 40 bar and − 120 °C, provided by a suitable refrigeration system. At such a low temperature, significant pre-treatment of the feed gas is required to prevent hydrate formation, solidification of CO2 and heat exchanger corrosion damage from mercury. In the current study, the conventional nitrogen removal unit (NRU) using CD is compared to the use of an optimized 9-step pressure vacuum swing adsorption (PVSA) detailed process model employing a Takeda carbon molecular sieve as the adsorbent. The PVSA unit preferentially adsorbs N2 based upon kinetic selectivity of the carbon molecular sieve. Nitrogen rejection by PVSA allows a higher tolerance to the contaminant levels over CD. The impact of this advantage on the overall performance of the entire natural gas upgrading process is evaluated by analysing, in HYSYS, the unit operations upstream and downstream of the CD unit. By employing dimethylethanolamine solvent for acid gas scrubbing and relocating the C2+ fractionation unit upstream of the NRU, the natural gas upgrading process is shown to be more profitable for the production of pipeline gas from raw natural gas at flow rates of up to 40 MMscfd (versus the currently accepted breakeven at 15 MMscfd) using PVSA instead of CD for the NRU.